What's the benefit, if nothing changes after they are held?
There are plenty of examples of situations where a disaster exercise is held before an actual event and the similarities are uncanny. See my previous blog post on that topic, COVID-19: Exercises Hurricane Pam and Crimson Contagion -- What They Have in Common.
Here is a brief article that touches on several elements of pandemic exercises, Pandemic Drill: Unanswered Questions.
As for the British Exercise Cygnus, keeping the results of an exercise quiet is the exact opposite for what should happen. Likely the "powers that be" thought the results showing the lack of readiness would be called out as a deficiency that they had no answer for.